Support Homebrew and XQuartz on Mac
MacPorts is currently preferred by NetRadiant, but it doesn't run anymore (at least, I couldn't get past a Segmentation fault: 11 error).
Homebrew moved to XQuartz a while ago, replacing X11 as the rendering system. NetRadiant still contains some hacks for X11, like GDK_DISPLAY(). Any chance someone could fix these hacks and make it run with XQuartz/Homebrew?